Framus Cobra issue: NOISE/CRACKLING, Advice needed

[ocram]

Hi everybody, I'm posting here because of a problem happening with my amp, a Framus Cobra 100w Head. Sincerily this problem begun not too much time after I bought it, let's say more or less one year (amp is 5 years old). I first ignored it because I always blamed the genre I was playing (fast deathcore) and the amount of gain used. Lately (during the last year) I found myself needing a less extreme sound so I was quite disappointed noticing that even with small gain amounts the problem was still there.
I searched plenty of forums/threads with similar problems, most say it's a matter of tubes, but after some diy troubleshooting I can say that I'm 99% sure I ain't got a power amp problem, but a Preamp section problem (even tho tubes aren't the cause IMO). Let me explain the problem:

A constant noise, similar (but smoother) to white noise, coming up eventually with pops, cracks and fried-bacon sounds in all of the 3 channels, Crunch and Lead channels being affected much more by it, but still present on the clean channel.
http://soundcloud.com/mmse/cobra-crackling-noise

Every channel has a gain knob and a volume ( output ) knob before entering the power amp section. Gain affects the quantity of noise, even if at 0 gain I can still notice the problem. Output knob totally kills the noise if set at 0, plus if i connect a jack to the "return fx" with the mix level set at "100%" the noise is gone too. That's why I'm saying the issue must be in the preamp. (according to schematics v4 is effects path and v5 is phase inverter, so if I'm not mistaking the thing i did with the "return" jack ensures that v5 and everything ahead is fine).
Read everywhere that 90% this is a preamp tube problem, changed them all with no luck. Tried cleaning and retensioning sockets, no luck.
This thing appears to happen very randomly in intensity, sometimes it's less significant and sometimes it seems like the amp is going to start sparkling and blowing itself up.

More weird things also happened..
Before changing tubes it happened 2 or three times to find myself stuck with no volume at all while playing, with just a 50Hz hum coming from the speakers. "restarting" the amp solved the problem. Both times the failure was anticipated by more or less 5 mins of an ugly-overcompressed-distorted sound.
Today it even did another crazy thing. I turned the amp on and the 2 overdriven channels didn't work. Swapped v2(new) with an old one solved. Tried that v2(new) in other positions and the tube just failed everywhere.. tried that v2(new) again after 8 hours and the tube is just fine (noise/cracks/pops still there).

So guys... anyone here have a clue of what's going on here??It's really making me crazy, I'm not a tech so I can't go further than swapping preamp tubes...
I need to go on tour in 4 weeks and it would be also nice to know if in your opinion it could be something easy and quick or something that could require more time...if so I need to start looking around for another amp..

Thanks in advance for your help!!

[ICBM]

First, welcome to Musicradar and well done for detailing the problem so thoroughly . (Not just "my amp has stopped working, does anyone know what's wrong?" )

It's difficult to be sure without actually being hands-on with the amp, but that sounds like it could be a faulty plate resistor, probably very early in the circuit if the noise it generates is more amplified by the dirty channels, and if it makes the whoe amp cut out.

I don't have the schematic, but is V1 (or at least half of it) common to all three channels? If so, I'd try changing the plate resistor for it. I would also *not* use a carbon-comp type resistor, if the original one is - they are used for "tone" but are more prone to problems like this. Carbon-film is more reliable - and if there is a real difference in tone, it's unlikely you'll hear any from just changing one resistor. I would also use the "next up" power rating if it will physically fit on the board.

It could still be many other things, including other failed/failing components or dray solder joints etc, but I would start there.

[paultheoneyoulove]

Wow, that's a whole bunch of issues. It sounds like faulty components within the circuit - I'm not a tech myself, but I had a similar problem with my old Laney. It needed 2 new power tubes, a preamp tube and a few other little bits and bobs replacing. Take it to a pro tech - it'll cost a fair bit, but at least you know you'll get your amp back working.

Also, don't go on tour without a backup amp! It would be a nightmare if this one went en route... Choosing a backup is tricky though, as reliability (solid state) tends to lose tone. Just be happy that to the average layman, one amp much sounds like the next!

[musicegbdf]

Welcome to the Forum and hope you stay around even once you get the problem fixed.
It is worth it , there are seem really good guys on here. There are also some real Guru's on specific needs as we have Guitar Builders/Amp builders/Pedal Builders all on here who give great advice.

ICBM is one of the amp Guru's so what he said , and if you are lucky I am sure a few others will pop up...

There are not many disputes on here as long as you don't mention the dreaded word (say it quietly ...Relics ).

[ocram]

Hey guys, thanks for the welcoming!!!
Thanks ICBM, your reply has been the most useful so far, and even tho I'm not a tech myself, I think you're right saying it might be something very early in the circuit..

http://www.framus.de/media/manuals/A...a/Cobra_V5.pdf

Indeed half of the v1 is the input stage,(at least that's what I can decode from the schematics, tell me if I'm wrong) and it's common in all of the three channels...
will go to a tech today, pointing out the troubleshooting you did and will see what he says...

Paul, I will for sure take another amp with me on tour, I'll borrow it from a friend ( a framus Dragon ).. luckily the 2 amps are very similar, so I won't be too homesick

[ICBM]

Thanks for the schematic. Yes, V1A is common to all three channels. Almost nothing else in the preamp is, so the fault is almost certainly associated with it.

The plate resistor for V1A is R3, which I also see is labeled 221K - much higher than the standard 100K value traditionally used - and which is common on high-gain amps, and fairly commonly a cause of trouble, because the higher value results in a greater voltage drop across it; this can exceed the rating of lower-power-rated resistors and is what causes them to fail.

If this is the cause, using a higher-rated resistor would definitely be a good idea.

[ocram]

Amp back from the tech, unluckily the problem persist.
He said he couldn't find the real cause of the noise, he just supposes that it may be some faulty integrated circuit on the board ( 6-pin things, maybe optocouplers?? )...

He did the bias of the power tubes, only positive thing is that now the amp sounds louder at less "knob" volume, so the noise is now a minor issue while playing.

He also did a test of the "old" preamp tubes and said they're fine and I can keep them as good spares if needed.


Tho I'm still experiencing strange things:
Turned the amp on, and it had almost no volume and no gain in all of the 3 channels. Turning the volume knob on the guitar would give me A LOT of crackling/oxid-like noise. Noise (fried like) was present and consistent. Swapped the v1 with a spare one and it got way better. (gtr volume knob ok, volume and gain fine).
Then i noticed the noise would be present also with a 100% wet on the fx loop, thus making me think about a problem on the phase inverter ( tube or anything??)... I was playing and then after almost 1 hour the amp volume dropped almost to 0, so I shut off the amp. After shutting off the power, I heard strange popping sounds from the cabinet for like 3 seconds ( discharging/faulty caps?!? - I'm sorry, I dont know what it could be, just wondering ). Turned it back on, and all I could hear from the amp was a 50hz hum. Swapped v5 with a spare one and the amp worked (for an hour so far).

All this is really frustrating, is it possible that so many new/not old tubes fail randomly like that and then work fine?! If it was every other component, would the problem get better/change just swapping tubes?
I'm really hoping the problem gets costant now, because I can't bring to a tech and stay there and play for 2 hours hoping the problem will happen. Problem is that the problem doesn't seem to get consistent in time...
And I can't rely on this amp to play live because of the shame it would cause if it shuts off during a show..

[ICBM]

Did your tech actually change R3? I think it remains the most likely source of the noise, although possibly not the cutting out if changing V5 fixed that.

Crackling when adjusting the guitar volume is definitely an issue with V1A - it's leaking DC down the guitar cable. This is just down to a bad valve, but that could also be the cause of noise and loss of gain from V1A.

Yes, new valves can certainly be faulty, and if you found replacing V5 fixed the cutting-out problem you may have more than one... which would be unlucky, but not impossible.

What brand are your new valves? There are some Russian-made 12AX7s which have a very specific failure problem when used in 'cathode follower' positions - which V1 and V5 aren't, but V3 is... so if you've tried the valve there it could apply.

[jpfamps]

Amp back from the tech, unluckily the problem persist.
He said he couldn't find the real cause of the noise, he just supposes that it may be some faulty integrated circuit on the board ( 6-pin things, maybe optocouplers?? )...

This fairly poor show, as it can only be in a fairly restricted part of the circuit.

What tests did he do?

[ocram]

Hi ICBM, no he didn't change R3, I will definetly bring it to another tech and see what he says first, then if he comes up with nothing again I will ask him to do that... I think it can't hurt anyways...

The tubes i bought are JJ's 12AX7.

hi jpfamps, I don't have any idea of the tests he did, it sounded quite weird to me by the way... a sort of an excuse for not solving the problem "...I guess the problem is caused by ALL these components that are noisy, and since they're all scattered through the Preamp, the noise amplifies itself..."

quite cheap mh?

thanks for the replies guys!!